In a way you can consider poker 0 sum or positive sum depending on the utility you derive from the enjoyment of gambling. But that should also factor in the negative utility from gamblers that lose
edit: The economy being positive sum has nothing to do with the way wealth is distributed.
Income inequality does not imply zero-sum.
Compare this to something like trading apples and oranges, where one person gets an orange (maybe they are tired of apples) and the other gets an apple (maybe they are tired of oranges). Both gets something they want in exchange for something they don't want.
I get the sense that you are dismissing the value of entertainment in this exchange.
Many of the donators in a given player pool know that they are losers and have a rough budget of what they are willing to lose. They engage in the exchange because they value the entertainment.
I personally see poker games as a transfer of wealth from worse players to better players, with the house taking a cut for creating the market. At higher stakes where most solid pros play, this rake is a small percentage of the wealth transfer.
Note that for many of the losing players, substitute activities for poker are gambling in the pit and/or sports betting. Are these also not part of the economy? What about movies, concerts, and TV? There is value in creating experiences instead of creating things, and poker is an experience that some people like.
To be fair, one of the things that poker is good at is letting people believe they are winners when they are not — this facilitation self-delusion sometimes makes for a bad look. That said, the bank account doesn’t lie, and I’ve seen plenty of avid poker players quit or drop stakes dramatically while choosing a different hobby or form of entertainment.